In addition to actual paper machines, the paper industry utilizes various post-processing machinery, such as intermediate winders, coaters, different kinds of calenders, slitters, and rewinders. It is a characteristic of such post-processing machinery that at least one of the electric drives thereof is a machine called a center winder wherein a roll of paper is unwound or wound while the diameter of the roll of paper changes during winding. Another characteristic of the unwinders and winders is that the web being wound is controlled by means of tightness control. The tightness control consists of a controller whose current value is measured from the paper web by a sensor installed in a roll, as well as from a feedforward term wherein one part of the term consists of acceleratable moments of inertia.
The unwinders and winders of the post-processing machinery used in the paper industry include accelerating and decelerating large inertia masses, and in order to ensure successful winding it is important to know these inertia masses accurately. When determining the moment of inertia, it is important to know the density of the paper in the roll of paper. It is known to obtain the density of the paper from a data file concerning the roll of paper about to come off the paper machine. In the post-processing machines, density data may be obtained automatically from the paper roll data, or an operator has to manually enter the data into the system of the post-processing machine.
In connection with a grade change on a paper machine, or in other failure situations therein, the density may be erroneous or the data has been entered or measured incorrectly, in which case during post-processing the web tightness shows large variations that, at their worst, may cause the web to break and may thereby cause production losses. Erroneous density data may also cause the quality of post-processed rolls to be lower. A paper roll of poor-quality makes them more difficult to handle for printing machines, for example.
It is known to correct a density value by visually monitoring the process and the tightness control as well as the rolls of paper just off the paper machine. However, correcting the density value in such a manner requires great skill and experience. It is also obvious that such an empirical way of correcting the density value is a particularly unreliable procedure.